[media] tuner-core: Better implement standby mode

In the past, T_STANDBY were used on devices with a separate radio tuner to
mark a tuner that were disabled. With the time, it got newer meanings.

Also, due to a bug at the logic, the driver might incorrectly return
T_STANDBY to userspace.

So, instead of keeping the abuse, just use a boolean for storing
such information.

We can't remove T_STANDBY yet, as this is used on two other drivers. A
latter patch will address its usage outside tuner-core.

Thanks-to: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>